Resolves #266 and improve saved log likelihoods and priors

- For the emcee, ptemcee and cpnest samplers. Adds the log_likelihood
  and log_prior per sample to the posterior (for CPNest this was already
  there, but was named differently).
- For all other samplers, the log_prior is calculated after the run
  based on the samples.
- Modified the `prior.PriorDict.ln_prob` method to include an axis
  argument